Sicilian Dragon, 8... Be6 why bad?

Ok, so after the sicilian dragon, and the typical 6. Be6 Bg7 7. f3 0-0 8. Qd2 people almost always play 8... Nc6.

But why not 8... Be6? Obviously there is a reason why it's not as good, however I ask this because after 8... Nc6 9. g4 blacks common reply is 9... Be6, where all of the sudden it becomes a good move!

So what's the difference between those move orders, and Be6? THANKS!!! 

Also, Be6 isn't just a blind move. It covers c4 preventing white from moving there with the bishop, which was whites next move. It also helps attack whites king in the future. And, it cannot be delayed, because after Nc6 white will play Bc4 preventing Be6

After 9.g4 Be6 isn't so bad because if 10.Nxe6 fxe6 Black gets pressure on the backwards f pawn.

Thanks!